Eukaryotic translation initiation is a complex and highly regulated process by which the ribosome is initiated in protein synthesis. This process engages a large number of factors that organize subunit dissociation, recruitment of the small ribosomal subunit to the mRNA, placement of the Met-tRNAi into the P- site, scanning of the 5'UTR, start-codon selection, and subunit joining. The efficiency of translation affects the concentrations of synthesized proteins, which is crucial for processes, such as progress through cell cycle, onset of apoptosis, progress of development, or cell transformation. Translation initiation is modulated by properties of the mRNA, such as elements located in the untranslated regions, binding of regulatory proteins, signaling events, or environmental factors. Mechanisms of eukaryotic translation initiation are only partially understood at a molecular level. Our previous research has made important contributions to this field including structure determinations of the cap-binding protein elF4E, its complex with elF4G, discovery of inhibitors of the elF4E/elF4G interaction, structures of elF1A and elF1and identification of their locations on the 40S ribosomal particle, the structure determination of elF2a and discovery of the mechanism of its interaction with e!F2y and Met-tRNAi. Here we propose to continue this successful research pursuing three specific aims: Aim 1: Unveil mechanisms of mRNA recruitment to the 40S ribosomal particle through cap-binding proteins. Aim 2: Elucidate mechanisms of initiation complex assembly and start-codon selection through studies of the interactions of the initiation factors elF1, elF1A, elF5, elFSB and elF2. Aim 3: Characterize the role of the ATP-dependent RNA helicase elF4A in translation initiation.